Big Box Retail Stores, Inc. fires its employee Craig for criticizing his manager Daria in posts on social media forums. According to the National Labor Relations Board, this discharge is

an ethical issue only

Explanation:

According to the National Labor Relations Board, a company could fire its employee if such employee violates the company's ethical behaviour that is expected of him or her and such would be regarded as ethical issue only.

Most companies usually have ethics in which their employees are to abide with. The contents of the ethics are usually spelt out for all the employees and are bound by it. Where an employee violates any of the company's ethics, such could be disciplined in line with the appropriate term that is contained in the ethics manual.